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Pentaquark hunt draws a blank

An oversized subatomic particle that has challenged models of quantum physics since its reported discovery in 2003 does not exist after all, a new study suggests. But in trying to track the particle down, scientists say they have stumbled on important new insights about the forces that bind the building blocks of matter.

The short-lived particle – called a pentaquark – was thought to consist of five subatomic particles called quarks. Quarks normally only associate in groups of two – producing short-lived mesons, or three – producing the protons and neutrons that make up the bulk of normal matter.

But in 1997, researchers in Russia calculated the properties of a hypothetical pentaquark. They convinced an experimental team at a Japanese particle accelerator to look for the products expected when pentaquarks decay and in 2003 the team reported finding them.

That surprise discovery led other researchers around the world to scour their archived data for evidence of the pentaquark. Within months, a dozen teams announced seeing the particle, with a couple of groups even claiming to have discovered two additional types of pentaquark.

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But just as quickly, cracks began to appear in both the theory and observations of pentaquarks. About a dozen other teams failed to find the particles in their data, prompting theorists to wonder whether pentaquarks could only be created under certain experimental conditions.

Delayed decay

Even when experiments did detect it, different groups reported slightly different values for its mass. But, more alarmingly, all of the measurements suggested the particle took about 100 times longer to decay than other particles of its mass – about 1.5 times the mass of a proton.

“The theory community – myself included – became rather troubled about the particle,” says Bob Jaffe, a theoretical physicist at the Massachusetts Institute of Technology in Cambridge, US.

Now, an international group called the CLAS collaboration has used the Thomas Jefferson National Accelerator Facility in Virginia, US, to search for pentaquarks by shooting energetic photons at liquid hydrogen. The experiment found no evidence for pentaquarks, even though its design was similar to an experiment which had previously produced positive results, carried out by a German team called SAPHIR.

CLAS produced a result 50 times more precise than SAPHIR’s by collecting more than 10 times as much data at the expected energy range of the decay particles. “They put together a sophisticated experiment with high statistics,” Jaffe told New Scientist. “They watched for a long time and didn’t see it – and they should have”, if the pentaquark had been there.

Focused hunt

Tom Cohen, a theoretical physicist at the University of Maryland in College Park, US, agrees the new result is impressive. He says previous experiments were not specifically designed to hunt for the pentaquark and provided only limited data. That data was typically cut down even further to include only specific decay scenarios.

Those cuts, Cohen told New Scientist, lower the signal-to-noise ratio. So a few random particles in the expected energy range of the pentaquark’s decay detritus can look like a signal, he explains. “If you’re flipping coins, occasionally you’ll get a few heads in a row.”

The results from another pentaquark-hunting experiment at the Jefferson Lab in Virginia, US, are expected later in 2005. But even if that search turns up nothing, some scientists are anything but disappointed.

“I’m actually delighted,” says Jaffe. He and MIT colleague Frank Wilczek, who won the 2004 Nobel Prize in Physics for other work on quarks, were inspired to re-examine the forces that bind quarks so tightly together they are never found alone in nature. They developed a model in which quarks with different properties feel an attractive force to pair up, like the pairs of electrons responsible for superconductivity.

“Originally, we proposed that was the force that was binding the [pentaquark],” says Jaffe, adding that the latest result suggests the force is not strong enough to unite five quarks. “But along the way, we discovered other things the force is responsible for,” he says, explaining it appears to act in mesons and inside the trios of quarks that make up protons and neutrons.

CLAS team member Raffaella De Vita of Italy’s National Institute of Nuclear Physics presented the new results on Saturday at a meeting of the American Physical Society in Tampa, Florida, US.